This invention relates to a unidirectional solidified cast article having a columnar crystalline microstructure. In particular the invention relates to a cast superalloy article having at least one columnar crystal that is substantially free of defects. The invention further relates to a casting method to produce the cast article. Still yet, the invention relates to gas turbines having unidirectional solidified cast articles, such as blades, buckets, nozzles, vanes, and airfoils.
The mechanical properties of cast superalloy articles improve by applying directional casting techniques to produce columnar polycrystalline or single crystal articles. Single crystal articles differ from polycrystalline articles primarily by the absence of boundaries between differently or arbitrarily oriented crystals. Both single crystal and polycrystalline articles can have a columnar structure.
Directional casting techniques used to manufacture single crystal and polycrystalline articles start with a mold shaped to produce the desired cast article. One such process of manufacturing columnar single crystal and polycrystalline cast articles employs a Bridgman-type furnace and comprises the pouring of molten metal into a mold within a heated zone. A chill plate cools the base of the mold (water-cooled). Subsequent crystallization of the molten metal occurs by gradually withdrawing the mold from the heated zone. Convection and/or radiation cools the mold from the bottom and then upward to solidify the cast metal. Another process for making directionally solidified cast articles comprises pouring molten metal into a superheated mold situated in a heated zone and withdrawing the mold from the furnace into a liquid coolant bath. The coolant bath has a temperature lower than the solidus temperature of the cast superalloy metal.
While casting vendors use variations of both casting processes today, the quality and structure of the unidirectional cast article still needs improvement. There is a sensitive dependence of the mechanical properties on the grain structures of cast materials. The mechanical integrity of columnar single crystal and polycrystalline cast articles is dependent on the elimination of high-angle grain boundaries and equiaxed grains. Also, the cast articles having a length greater than about four inches, such as nozzles, buckets, or airfoils used in land-based turbine generators, generally exhibit substantial interdendrite segregation formed during the directional solidification process. Depending on the particular superalloy chemistry, the segregation can result in the formation of low melting point or brittle phases, nonuniform distribution of strengthening precipitates, interdendritic porosity, and surface freckles. The term "freckles" or "freckling" means that during solidification of superalloy columnar single crystal or polycrystalline cast articles chains of very small equiaxed grains form. It is proposed that in directional solidification, where the liquid melt is maintained above the solid, these chains of freckle type defects develop when segregating elements alter the liquid density of the interdendritic fluid to a sufficient degree to initiate a convective instability. One or more of these structural manifestation can be undesirable. Further, the methods for minimizing the presence or effects of dendrite segregation, including solid state diffusion heat treatments or mechanical working, are not feasible for use with complex alloys or large cast articles.
Dendrites formed within the columnar single crystal or polycrystalline article are distinguished from the surrounding material by differences in concentration of some constituents. Embedded particles and elemental microconstituents of the alloy chemistry tend to accumulate in the normally weaker interdendrite regions. As a result the strength of the cast alloy is decreased by such inhomogeneities. The size of the embedded particles and pools of the microconstituents is significantly reduced by a reduction in primary dendrite arm spacing in the cast article. The primary spacing is the average spacing between adjacent dendrite cores. Primary dendrite arm spacing is measured by sectioning normal to the crystal growth direction, counting the number of primary arms over the cross-sectional area, and calculating an average spacing. Typically, average spacing is determined assuming a square array. Secondary dendrite arm spacing is the average spacing between adjacent secondary dendrite arms as observed on a section containing the growth direction. Thus, there is a need to produce unidirectional cast articles with minimal primary and secondary dendrite arm spacing to achieve superior mechanical and chemical properties with decreased structural defects.
Dendrite arm spacing is also a measure of the solidification conditions of a casting. Dendrite arm spacing varies inversely with cooling rate (solidification rate times thermal gradient). High thermal gradients are required to prevent nucleation of new grains during directional solidification; high cooling rates are required to prevent freckle formation.
Hitachi, in U.S. Pat. No. 5,489,194, addresses the casting of single crystal nickel superalloy blades for turbines that are seven inches or greater in length. Hitachi obtains single crystal microstructure in a blade comprising a dovetail with a shank being connected to the dovetail and having one or more protrusions formed on the side of the dovetail, and with a vane being connected to the shank. Because of the use of protrusions in a by-pass mold, Hitachi forms a large single crystal blade. The casting process is performed in a conventional Bridgman furnace using a chill plate with radiant and convection cooling. However, Hitachi does not teach or suggest fine dendrite spacing in the single crystal blade. In fact, although Hitachi produces a large single crystal blade of about 160 mm (6-7 inches in length), the Hitachi single crystal structure is expected to have large dendrite arm spacing due to the low cooling rates of radiation from a mold to the walls of the furnace. Also, after casting the single crystal blade, Hitachi subjects the blade to a solution heat treatment, followed by an aging treatment. The various heat treatments take several hours. Hitachi's blade, while single crystal, still does not solve the problem of obtaining fine primary dendrite arm spacing to provide an homogeneous microstructure with improved mechanical properties in large cast articles. FIG. 1 shows a plot for dendrite arm spacing 40 versus the size of the cast article obtained by conventional casting methods such as used by Hitachi with vacuum radiation cooling.
Since Hitachi's blade is cast by the conventionally cooled method, the cooling rate or thermal gradient is a sensitive function of the size of the blade to be cast. As a general rule of thumb, the cooling rate or thermal gradient is inversely proportional to the blade size. When the size of the blade increases, the cooling rate and thermal gradient decreases, and the tendency of extraneous grain nucleation increases. The types of grain defects caused by the reduced cooling or thermal gradient in large blades include those known in the trade as freckles or slivers. These types of defects, once formed due to the reduced thermal gradient, are not restricted to protruded areas of the blade such as platform or angle wing. Due to this unpredictability, the by-pass mold designed to eliminate grain defects in the shank area, as discussed in the Hitachi patent, will not be effective in producing a totally defect-free large blade. Even with the by-pass mold, Hitachi's blade will be difficult to cast free of defects.
On the other hand, U.S. Pat. No. 3,915,761, discloses a superalloy cast blade for aircraft engines that is about four inches in length (col. 6, lines 5-6; col. 9, lines 23-24) with a hyperfine primary dendrite spacing of less than about 0.005 inches or 130 micrometers (.mu.m). Herein, "hyperfine" primary dendrite spacing means average spacing less than 0.005 inches (130 .mu.m) between adjacent dendrite cores. The hyperfine dendrite spacing is accomplished by using a casting method that utilizes a liquid cooling bath that provides a high solidification rate by withdrawal of the part from the furnace at about 120 inches per hour. This teaching is limited to aircraft size parts and has not been demonstrated for land-base turbine components. In fact, the size of land-base turbine parts prohibits the withdrawal rates used in '761.
U.S. Pat. No. 3,915,761 requires "hyperfine" primary dendrite spacing, attributes not achievable in large cast parts which are about seven inches in length or greater. This is partially due to the large size and its cross-section.
Large cast parts of defect-free columnar structures would be of great benefit for large gas turbines. For instance, consider the thermal efficiency of gas turbines as an important measurement of the performance of a power generation engine. An efficient engine is typically run at a high enough temperature so that the fuel energy can be effectively utilized to generate low cost electricity. New generations of power generators will require larger turbine capacity and component sizes. Blades that are twelve inches or greater will be required. However, a limitation of gas turbines is the availability of turbine articles that can sustain high temperature and stress in the engine environment. To cope with such an increase in the gas temperature, conventional cast articles, such as buckets, blades, nozzles, vanes, and airfoils have complicated geometry's and cooling holes. This further poses problems in the casting operations utilized to make the article as well as the ability to provide the required mechanical and chemical properties of the cast article.
For these reasons, there is a need for a large unidirectional solidified columnar cast article that is single crystal, polycrystalline, or a mixture of single and polycrystalline microstructure that is substantially defect free, without requiring the impractical hyperfine dendrite arm spacing 30 of U.S. Pat. No. 3,915,761 as displayed in FIG. 1. The fine dendrite arm spaces 50 shown in FIG. 2 in large unidirectional columnar cast articles provides improved chemical and mechanical properties of the cast article.